This invention relates to a variable-voltage variable-frequency (VVVF) elevator control apparatus, and in particular to a VVVF elevator control apparatus having improved ability to handle regenerated electric power produced during emergency operation of the elevator.
First, a conventional VVVF elevator control apparatus will be described with reference to FIG. 1 of the attached drawings. In the figure, element number 1 is a 3-phase AC main power source, element number 2 is a relay energized by the main power source 1 for detecting any cutoff of power from the main power source 1, element number 3 is a 3-phase AC emergency generating unit comprising a Diesel engine E.G. and an emergency generator G, element number 2A is a normally open contact (a contact which is open when the relay 2 is not energized and which is closed when it is) of the relay 2 connected in series in the load line of the main power source 1, and element number 2B is a normally closed contact (a contact which is closed when the relay 2 is not energized and which is open when it is) of the relay 2 which is connected in series in the load line of the emergency generating unit 3. FIG. 1 represents the state when the main power source 1 is off and accordingly the relay 2 is not energized. Thus, the figure shows contact 2A in its open state and contact 2B in its closed state. Element number 4 is a power running converter, i.e. a 3-phase full-wave rectifier comprising a 3-phase diode bridge. The AC input side of the power running converter 3 is connected to the main power source 1 via the normally open contact 2A and to the emergency generating unit 3 via the normally closed contact 2B. Element number 5 and element number 6 are a reactor and a smoothing capacitor, respectively, connected on the DC output side of the power running converter 4. Element number 7 is an inverter of the pulse width modulation (PWM) type comprising a transistor bridge. Element number 8 is a 3-phase induction motor connected to the AC output side of the inverter 7 which is used to power the elevator. Element number 9 is a regenerated power AC-DC converter comprising a thyristor bridge. The regenerated power converter 9 has its DC input side connected to the DC side of the power running converter 4 and has its AC output side connected to the AC load line via a boosting transformer 10. Element number 11 is a regenerated power consumption circuit connected to the AC load line, the consumption circuit 11 comprising thyristors 11A, resistors 11B, and diodes 11C. Element number 13 is a tachometer generator for measuring the speed of the induction motor 8, element number 14 is a voltage detector for detecting DC voltage, and element number 20 is a speed command generator which outputs a speed command to a controller 21 which outputs control signals to the inverter 7, regenerated power converter 9, and thyristor 11A. The controller 21 is connected to a normally closed contact 2C (one which is closed when the relay is not energized and which is open when it is) of the relay 2 for detecting power cutoffs.
During normal power running of the induction electric motor 8 in an elevator controlled by this conventional control apparatus, the relay 2 is energized by the main power source 1, and the normally open contact 2A is held closed while the normally closed contact 2B is held open. (In the description below, "normal" will be used to refer to the state when the main power source 1 is in operation, and "emergency" will be used to refer to the state when the main power source 1 is disconnected and the emergency generating unit 3 is in operation.) AC power from the main power source 1 is full-wave rectified by the power running converter 4. The DC output from the power running converter 4 is smoothed by the smoothing capacitor 6 and then converted to 3-phase AC power by the inverter 7 which produces a variable voltage and variable frequency by pulse width modulation, and this AC power is then applied to the induction motor 8 to drive it.
During normal regenerative running of the elevator, the thyristors in the regenerated power converter 9 are turned on by a signal from the controller 21, and the regenerated power converter receives regenerated power from the induction motor 8 which it returns to the main power source 1 via the boosting transformer 10. At this time, if the reactance of the reactor 5 is sufficiently large, the power returned to the main power source 1 is a square wave with a 120.degree. conduction period. Since the thyristors 11A are not turned on at this time, the consumption circuit 11 does not absorb any of the regenerated power.
Next, the operation of this conventional control apparatus will be explained for the case when power is provided by the emergency generating unit 3. If for some reason the main power source 1 should fail to function, the relay 2 for detecting power cutoffs will drop out, causing the normally open contact 2A to open and the normally closed contact 2B to close. This will allow the emergency generating unit 3 to supply power to the induction motor 8. The power produced by the emergency generating unit 3 is full-wave rectified by the power running converter 4, smoothed by the smoothing capacitor 6, converted to pulse width modulated 3-phase AC power by the inverter 7, and applied to the induction motor 8 to drive it.
In general, the capacity of the emergency generating unit 3 is quite small, making it impossible for it to absorb the power produced by the induction motor 8 during regenerative braking. Accordingly, the regenerated power consumption circuit 11 is provided in the conventional control apparatus to absorb this power. The thyristors 11A in the consumption circuit 11 may be used merely as a switch or may be linked with the gating of the thyristors of the regenerated power converter 9 and be phase controlled. The signal for gating the thyristors 11A is issued by the controller 21. The internal logic of the controller 21 is switched when the normally closed contact 2C closes upon the occurrence of a power cutoff.
Because the thyristors comprising the regenerated power converter 9 are phase controlled, an overlapping angle is produced during thyristor commutation and a sharp voltage notch appears. This voltage notch may cause the exciting gating circuit (not shown) for the emergency generating unit 3 to misfire which results in the unstable operation of the emergency generating unit 3. This conventional control circuit thus has the disadvantage that it is necessary to increase the capacity of the emergency generating unit 3 in order to reduce the effect exerted upon it by voltage notches.
Another disadvantage of this conventional control apparatus is that the regenerated power consumption circuit 11 is on the AC side of the converter 4 and must absorb 3-phase AC power. Accordingly, the circuit 11 requires a large number of elements: at least three thyristors, where diodes, and three transistors.